The acoustical spectrum may be divided into three regions on the basis of the limits of hearing of the human ear. The region from 20 to 20,000 cycles per second is generally referred to as the sonic, the region below 20 cycles per second is the infrasonic, and the region above 20,000 cycles per second as the ultrasonic region.
Non-destructive testing and evaluation is one of the most important fields of passive applications of ultrasonics. In ultrasound material testing, measuring the velocity of an attenuated wave in an object under an examination is one of the most useful expedients for determining the physical and material properties of the object.
Even though ultrasonics has been extensively used in metallurgy for detecting flaws and monitoring thickness of metal objects; its application to a nondestructive examination (hereinafter NDE) of polymers has been limited. There are several reasons for the lack of wide scale acceptance of NDE ultrasonic methods. Firstly, the attenuation of ultrasonic signals in polymers is, in general, several orders of magnitude larger than that typically found in metals. Adaptation of existing NDE methods for the study of polymers is feasible but such an adaptation requires a sufficient demand for the development of special high power ultrasonic generators and ultra sensitive ultrasonic wave detection systems to make ultrasonic measurements feasible. Secondly, the type of data generated by the NDE ultrasonic method is not widely understood and hence potential problems associated with NDE are generally not recognized. Finally, the generally low cost of polymeric articles makes routine examination of such articles economically not feasible. As a result, there was little desire to develop such methods. However, in recent years, the advent of specialty polymers has led to the use of polymers in large structural applications and, hence, there is now a justification to take a new look at non-destructive testing methods in general and to the ultrasonic techniques in particular.
An ultrasonic wave may be used to determine the physical attributes such as Young's modulus, Poisson's ratio, shear modulus and volumetric fraction of fiber reinforcement present in the fiber reinforced polymer composites.
When any type of reinforcement, such as nonrandomized discontinuous fibers, is present in the composite medium, such a medium exhibits anisotropic behavior. Anisotropic behavior results in a medium having properties that differ according to the direction of measurement, whereas isotropic behavior results in a medium having properties that are identical in all directions. Such isotropic behavior is typically exhibited by a composite medium having a substantially randomized three dimensional discontinuous fiber distribution.
As their name implies, fiber reinforced composite materials (also known simply as "composites") comprise fibers of materials such as carbon, boron, graphite, glass, nylon, polyester or metals and their alloys such as steel, impregnated with a matrix or a polymer material such as epoxy resin. Such composites typically exhibit extremely high strength to weight ratios, and accordingly their use is becoming increasingly popular in the aerospace and automotive industry.
One of the problems still associated with discontinuous fiber composites is anisotropic flow behavior. As a result, knowing the fiber orientation within composites allows one to predict an optimal direction in which the best mechanical properties may be obtained.
Still another problem especially associated with discontinuous randomly distributed fiber reinforced composites is that it is difficult to determine the volumetric fraction occupied by the fibers in a composite matrix to a degree of certainty typically desired in a manufacturing process. Accordingly, new means are constantly being sought for determining the fiber orientation and volumetric fraction more efficiently and at a lower cost.
Discontinuous fiber composites with long (typically about 0.5 to about 1.0 inch length) fibers such as glass fibers are being used in many applications where stiffness, impact strength, and low costs are important. An example of a discontinuous glass fiber composite is sheet molding compound (SMC) which contains a thermosetting resin, glass fiber and filler. The glass fiber may be in the form of a randomly distributed planar discontinuous fiber or a woven fiber mat or a continuous fiber. Composites based on a thermoplastic resin are also available. Examples of such thermoplastic composites are those produced by a process involving a powdered resin, chopped glass fibers, water and a surfactant mixed to form an aqueous foam. The foam is poured on a filter screen where water is removed by vacuum, leaving behind a fibrous web or felt of chopped fibers and resin particles. The web is then laminated under heat and pressure to form a sheet. Such sheets are then workformed into desired articles. The term "workforming" is defined as a method by which the aforementioned sheet may be shaped, machined, or modified in a predetermined manner. A workforming apparatus, such as a thermostamping machine, is well known in the art.
Discontinuous fiber composites of this type may be produced by either a batch or a continuous process. The batch process can be manipulated to produce felts with either some degree of in-plane fiber orientation or a nearly two dimensional random fiber distribution. In the continuous process of this type, the aqueous foam is fed through a manifold onto a moving filter screen, which may result in some preferred orientation in the direction of the moving filter screen. Such preferred orientation may be controlled by modifying the manifold design and adjusting the process variables, such as filter screen speed and flow rate of the aqueous foam. The fiber orientation in the sheet results in anisotropic flow behavior. For example, FIG. 1 shows the effect of fiber orientation on the deformation behavior of a test sample made of 30% by weight of glass filled polypropylene. "A" of FIG. 1 shows the original shape of a test sample, an arrow indicating the direction of the fiber orientation, and "B" of FIG. 1 shows the test sample after its compression to 60% of original thickness when pressed between the parallel plates of a squeeze flow rheometer. As seen in "B", a significantly larger deformation occurs in a direction orthogonal to the direction of the fiber orientation.
Such flow anisotropy complicates the selection of thermoforming molding conditions and placement of a blank in the thermoforming mold. In addition, anisotropy in final parts can lead to unbalanced residual stresses that may be sufficient to warp the thermoformed articles. Such anisotropic behavior is not always undesirable; but if controlled and understood, it can lead to a better and more efficient thermoforming process. Hence, there is a need for simple nondestructive, quantitative techniques for characterizing material anisotropy and mechanical properties in these composites.
An ultrasonic wave propagating through a solid body can be used as a probe for finding material alterations throughout that body. The use of such a probe offers a distinct advantage over techniques that rely on surface measurements of the body, since many properties exhibited by the surface layer of the body are not identical with the behavior exhibited by the bulk material of the body. Ultrasonic material evaluation techniques have been used for elastic property characterization of both isotropic and anisotropic materials. Ultrasonic methods for the testing of polymers have been developed using liquid immersion techniques, solid buffer rod techniques or direct contact techniques.
Ultrasonic immersion techniques are well known in the art and such techniques involve placing a sample in the path of a sound wave between a transmitting transducer and an opposite receiving transducer both of which are immersed in a sound conducting fluid. Under certain conditions mostly a longitudinal wave may be generated by changing the orientation of the sample with respect to a path of a sound wave (when the sound wave is perpendicular to the plane of the sample) or mostly a shear wave may be generated when the angle of incidence exceeds a critical value for total internal reflection (based on Snell's Law). The shear wave is also called a transverse wave.
The immersion technique suffers from several drawbacks. As can be seen in U.S. Pat. No. 4,346,599, to McLaughlen et al, the apparatus requires a great number of mechanical and moving parts (e.g., motors, gears, chains, cams, etc.) which demand minute adjustments. Furthermore, before any measurements can be made, fine adjustments must be made to the components of the apparatus. Such adjustments are difficult to make and can lead to erroneous results if not performed properly. As such they constitute a source of measurement error. Also, in the immersion technique it is necessary to perform all measurements in a water tank and a complicated numerical analysis procedure is needed for modulus determination.
Ultrasonic material evaluation techniques based on solid buffer rod techniques are well known. In such techniques the sample is placed between two solid (metal or glass) rods. A first transducer acting as a transmitter is affixed to a free end of the first buffer rod and a second transducer acting as a receiver is attached to a free end of the second buffer rod. An ultrasound burst is generated by the first transducer which then travels through the first rod, the sample, the second rod, and finally to the receiver. A part of the energy of the ultrasonic burst is transmitted through the sample and then received by the receiver where it is detected. However, such techniques are unsuitable for determining the in-plane properties of the sample under analysis.
In still another method, the velocity of plate bending (Lamb) waves is measured from a single side of the sample using a contacting transducer assembly. However, a fairly accurate estimate of the composite Poisson's ratio is needed to calculate the moduli. The aforementioned method was limited to determinations of the elastic moduli along the principal fiber directions in composites containing unidirectional and crossply geometries. An additional limitation of this method is the necessity of solving complex mathematical equations, such as the Fourier transforms of stored pulse wave forms, due to a dispersive nature of the Lamb waves.
Finally, most of the aforementioned prior art methods for the determination of elastic moduli require prior knowledge of the sample density, which is not usually known in the fiber reinforced composites.